1. Field of the Invention
This invention relates to certain halo stabilizer precursors in photothermographic elements, compositions and processes to provide improved stability. In one of its aspects it relates to a photothermographic element comprising (a) photosensitive silver halide in association with (b) an oxidation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent with (ii) an organic reducing agent, (c) a polymeric binder, and (d) a certain halo organic stabilizer precursor as described herein. In another of its aspects it relates to a photothermographic composition comprising the described stabilizer precursors. A further aspect relates to a method of developing a stable image in a photothermographic element comprising the described oxidation-reduction image-forming combination with photosensitive silver halide and the described halo organic stabilizer precursors by overall heating the exposed photothermographic element.
2. Description of the State of the Art
It is known to obtain an image in a photosensitive element by so-called dry processing with heat. The photothermographic element employed for providing such an image can contain a reducing agent, a light-insensitive silver salt of an organic acid, such as silver behenate, as an oxidizing agent and a low concentration of photographic silver halide. Such photothermographic elements are described, for example, in U.S. Pat. No. 3,457,075 of Morgan et al., issued July 22, 1969; U.S. Pat. No. 3,152,904 of Sorensen et al., issued Oct. 13, 1964; British Pat. Specification No. 1,161,777 published Aug. 20, 1969; U.S. Pat. No. 3,707,377 of Tiers et al., issued Dec. 26, 1972 as well as in Research Disclosure, January, 1973, pages 16-21.
In a photothermographic material one of the main difficulties involves post-processing stability. Because heat developable photographic elements are suitable for so-called dry processing with heat and are designed to eliminate a fixing step which normally would remove undeveloped silver, it is necessary that a means be provided for post-processing stabilization to enable room-light handling.
Several means have been proposed to answer the need for post-processing stability of photothermographic elements, such as (1) washing with water to remove undeveloped silver salts, (2) heating to release Bronstead or Lewis acid such as HCL, fluoride or HF from incorporated compounds such as m-nitrobenzenesulfonyl chloride, para-toluenesulfonic acid urea addition complex or p-acetamidobenzenediazonium fluoroborate and (3) chelation of the oxidizing agent with, for example, salicylaldoxime or benzotriazole as described in U.S. Pat. No. 3,152,904 of Sorensen et al., issued Oct. 13, 1964. Another method proposed for solving the problem of post-processing instability is to provide a photothermographic combination in which the oxidation-reduction image-forming combination is on one sheet of material and the latent image forming photographic silver halide is on a separate sheet. The sheets are separated after exposure such as described in U.S. Pat. No. 3,152,904. A further means proposed for stabilization involves swabbing a 1% solution of phenylmercaptotetrazole onto the surface of the overall heated photothermographic material or rubbing benzotriazole into the surface of the coating. These proposed means for stabilization are not useful for large volume handling of heat developable photographic materials.
It has also been proposed to provide stabilized images in heat developable materials by treating the developed image with a solution containing certain thiol or thione compound stabilizers. This is described, for example, in U.S. Pat. No. 3,617,289 of Ohkubo et al., issued Nov. 2, 1971. One of the compounds proposed for this solution stabilization is 1-phenyl-5-mercaptotetrazole. Unfortunately, the addition of this compound to photographic silver compositions provides undesired desensitization at concentrations which produce stabilization and toning of a developed image.
Another means of stabilization of an image in a photothermographic material is described in U.S. Pat. No. 3,839,041 of Hiller, issued Oct. 1, 1974. Certain stabilizer precursors which are azole thioethers or blocked azolinethiones are employed in photothermographic materials according to the description in these applications. An example of a stabilizer precursor proposed in photothermographic materials is 5-methoxycarbonylthio-1-phenyltetrazole. Unfortunately, this compound does not always provide the desired increased stability of a heat developed image.
Another means for post-processing stabilization of certain photothermographic materials is described in U.S. Pat. No. 3,707,377 of Tiers et al., issued Dec. 26, 1972. Certain polyhalogenated organic oxidizing agents, such as tetrabromobutane, are described in certain heat developable photographic materials to provide post-processing stabilization. Unfortunately, multihalogenated image stabilizer precursors, such as tetrabromobutane, do not stabilize certain photothermographic materials as efficiently as monohalo compounds. This is illustrated by comparing the results in following Example 7 with results in following Example 11. Certain monohalo compounds can provide surprisingly improved stabilization without providing significantly decreased maximum density.
Halogenated organic compounds have been employed in photographic materials for various purposes, such as antifoggants. Monohalogenated organic antifoggants for photographic emulsions are described, for example, in U.S. Pat. No. 3,128,187 of Sagal et al., issued Apr. 7, 1964; U.S. Pat. No. 3,232,762 of Ford et al., issued Feb. 1, 1966; U.S. Pat. No. 2,732,303 of Morgan et al., issued Jan. 24, 1956; U.S. Pat. No. 2,835,581 of Tinker et al., issued May 20, 1958 and U.S. Pat. No. 3,271,154 of Dersch, issued Sept. 6, 1966.
There has been a continuing need for photothermographic materials, especially photothermographic materials comprising an image-forming combination containing photosensitive silver halide in association with an oxidation-reduction image-forming combination comprising an oxidizing agent comprising a silver salt of a long-chain fatty acid with an organic reducing agent and a polymeric binder, which has improved post-processing image stability, that is reduced background density and desired maximum density, when employing a halogen containing stabilizer precursor. There has also been a need to provide improved incubation antifoggants for such silver halide photothermographic materials.